a. Field of the Invention
This invention relates to methods and apparatuses for removing SO2 from a gas stream.
b. Description of the Related Art
Fossil fuels are burned in many industrial processes. Electric power producers, for example, burn large quantities of coal, oil, and natural gas. Sulfur dioxide (xe2x80x9cSO2xe2x80x9d) is one of the unwanted byproducts of burning any type of fossil fuel. It is known to cause acid rain, and to have serious negative health effects on people, animals, and plants. A great deal of research has been done to find a way to economically remove SO2 from flue gas streams before it enters the atmosphere.
SO2 is often removed from gas streams (xe2x80x9cdesulfurizationxe2x80x9d) by scrubbing the gas with an aqueous ammonium sulfate solution containing ammonia. Examples of this process are disclosed in U.S. Pat. Nos. 4,690,807, 5,362,458, 6,221,325, and 6,277,343, which are not admitted to be prior art by their mention in this Background section. The absorbed sulfur compounds react with ammonia to form ammonium sulfite and ammonium bisulfite, which are then oxidized to form ammonium sulfate and ammonium bisulfate. The ammonium bisulfate is further ammoniated to form ammonium sulfate.
The patents referenced above teach, among other things, that the pH of the ammonium sulfate solution should be kept between about four and six. This range is the result of a compromise between competing factors. On one hand, ammonium sulfate solution is capable of absorbing SO2 more rapidly when its pH is higher. The ability to absorb SO2 better implies that the size of the scrubbing tower can be smaller, thus saving capital costs. In addition, the liquid to gas (xe2x80x9cL/Gxe2x80x9d) ratio can be smaller, meaning less liquid will be required and operating costs will be lower.
On the other hand, higher pH levels are also associated with the release of free ammonia from solution, often termed xe2x80x9cammonia slip.xe2x80x9d In addition to incurring an economic loss because of lost ammonia, free ammonia in the scrubbed flue gas reacts with uncaptured sulfur dioxide and trioxide to create an ammonium sulfate/bisulfite aerosol that is visible as a blue or white plume in the stack discharge, leading to secondary pollution problems. Controlling the amount of free ammonia in the desulfurization process is in part a function of the ammonia vapor pressure, which results from a combination of pH and levels of unoxidized ammonium sulfite that remain in the absence of sufficient oxygen. Therefore, high pH values and high levels of unoxidized ammonium sulfite promote ammonia slip.
One exception is a method disclosed in U.S. Pat. No. 6,063,352 in which the pH is kept between 4.5 and 7. However, this method avoids the formation of aerosols by maintaining a very high ratio of ammonium sulfite to ammonium sulfate. The ratio is on the order of 15:1 to 3:1. Maintaining this ratio in a scrubber on a commercial scale would be very difficult.
In the past, the solution to the problem of removing SO2 from a gas stream prior to the present invention has been a compromise between these two competing factors: maintaining the pH of the ammonium sulfate solution to between about four and six. It is recognized that a pH of greater than six is likely to produce ammonia slip. If the aerosols could be removed from the gas stream, the SO2 removal process could be made much more efficient without the need for compromise. What is needed, therefore, is a process that removes SO2 from a gas stream that can occur at a pH greater than six, and minimizes ammonia slip.